The invention concerns a pulse code modulation, time division multiplex switching network of a PCM/TDM tandem exchange for the connection of receive PCM/TDM trunks to transmit PCM/TDM trunks, and the time allocation of time slots employed on the receive PCM/TDM trunks to time slots to be used on the transmit PCM/TDM trunks.
PCM/TDM switching networks of this type and of known construction are divided into TDM and space division multiplex (SDM) switching stages. The TDM switching stages have buffer storages in which the PCM signals are stored intermediately during the time period between the time position corresponding to the channel utilized on a receive PCM/TDM trunk coming into the exchange and the time position corresponding to the channel used on the transmit PCM trunk emanating from the exchange and associated with the connection involved. The SDM switching stages include crosspoints activated periodically by pulses, the receive TDM trunks being connected to the transmit TDM trunks over said crosspoints.
Various configurations may be formed in the above PCM/TDM switching networks of known construction through selection of the number of stages of the SDM switching stages and the succession of SDM and TDM switching stages. These various configurations are complex in their construction, and expensive, but their load-carrying capacity is relatively great.
With a growing number of PCM/TDM trunks to be connected, the use of a one-stage SDM switching stage rapidly leads to very large switching rows, consequently to a very large number of multiple-connected crosspoints. Therefore, such a configuration is, from the outset, not suitable for use in large exchanges. In the case of a two-stage SDM switching stage, the individual switching rows are smaller, but the number of links between groups of switching (or crosspoint) matrices associated with different switching stages must be smaller with a growing number of PCM/TDM trunks to be connected. In this case the danger of congestion increases if the switching network carries an uneven load from the PCM/TDM trunks.
If only a single TDM switching stage is employed for the construction of such a PCM/TDM switching network, the time position may be changed only once during the call setup, i.e., in every case either the time position seized on the receive TDM trunk or the time position which shall be seized on the transmit TDM trunk must be utilized in the SDM section of the switching network, thereby increasing the danger of time-slot congestions. This danger may be reduced by providing two TDM switching stages that enable a twice-repeated time-slot conversion, thereby rendering the switching in the SDM section of the switching network independent of the time positions on the TDM trunks, or by providing for expansion of the SDM repeater matrix. In both cases, however, the small probability of time-slot congestions is obtained at the price of greater expense.
Differences in the succession of TDM and SDM switching stages, likewise, lead to different traffic characteristics.
In a switching network in which the SDM switching stages are arranged between an input-side and an output-side TDM switching stage, as indicated hereinabove, the switching in the SDM section of the switching network is independent of the time positions utilized on the TDM trunks. However, "bottlenecks" occur at the inputs and outputs of the SDM switching network, since the possibility of switching an item of information on various SDM paths is solely provided within the SDM switching network. Thus, in order to avoid these bottlenecks, more components would also be required in this configuration, because of the requirement for expansion of the SDM switching network, such that several inputs and outputs of the SDM switching network are made available for each PCM/TDM trunk.
If the reversed configuration is selected, i.e., if a TDM switching stage is placed between an input-side and an output-side SDM switching stage, one is limited during the switching in the SDM section of the switching network to the time positions utilized on the PCM receive TDM trunk and on the PCM transmit TDM trunk which, as indicated hereinabove increases the danger of time-slot congestions. Moreover, the storage in such a centrally located TDM switching stage cannot be used as a buffer storage, so that for the equalization of delay times buffer storages allocated to individual lines must precede the input-side SDM section of the switching network.
Likewise, the alternate succession of several SDM or TDM switching stages does not solve the problems described hereinabove. As in the case of the stage succession mentioned hereinabove, one faces in multiple alternate stage successions the problem of bottlenecks occurring at the inputs and outputs of the SDM section of the switching network. That is, there is the time-congestion problem mentioned above with respect to the input/output-side SDM switching stages, and the bottleneck problem with respect to the central SDM switching stage.
The time-channel congestion problems in PCM switching networks of known construction are reduced in that operation in the exchanges takes place with a number of time positions greater than the number of the channels utilized on the TDM trunks. Since the power consumption in the PCM exchanges must retain restricted (rules having been laid down according to which it must not exceed 230 W per m.sup.2 of rack surface), with the present state of the art must resort to circuit techniques that do not tolerate operations at higher operating speeds or to operations that result in loss of time (e.g., a series-parallel conversion).
Bearing the above conditions in mind, it is an object of the invention to design a PCM/TDM switching network having better traffic characteristics and using fewer components than switching networks of known construction.